Torsional vibration damping mechanisms have long been used to reduce the adverse effects of torsional vibrations or fluctuating torques in vehicle drivelines. Such torsional vibrations or fluctuating torques, hereinafter referred to as torsionals, emanate primarily from engine power pulses and torque spikes, and from abrupt changes in driveline torque due primarily to rapid engine acceleration/deceleration and transmission ratio changes.
Most known, prior art torsional vibration damping mechanisms have employed springs disposed in parallel with a mechanical friction device. A well known and basic type of such mechanism has comprised plate like members mounted for limited relative rotation, a set of helical compression springs interconnecting the members and a mechanical friction device responsive to relative rotation of the members. Driveline torque is normally transmitted by the helical springs and flexing of the springs attenuates or reduces the potential amplitude of the driveline torsionals. The mechanical friction device dampens or reduces the rate of spring recoil. When the amplitude of the torsionals is less than the breakaway torque of the friction device, spring flexing does not occur and the torsionals are transmitted without benefit of attenuation.
It is also known to employ flat spiral wound or helical compression springs in parallel with a viscous coupling or damper mechanism, as may be seen by reference to U.S. Pat. Nos. 4,608,883 and 4,601,676, respectively, which are incorporated herein by reference. Since a liquid is the clutching medium within a viscous shear damper, the problem of breakaway torque associated with mechanical friction devices is in theory eliminated. However, such viscous shear dampers have been difficult to fit into the limited space available in vehicle drivelines and when reduced to sizes that fit in the limited space available, they have been difficult to assemble and properly fill with viscous shear oil, and they have required costly and/or bulky dynamic seals to eliminate seal drag torque. Further, when the housings of the viscous shear devices have been formed of relatively thin walled stampings to reduce the axial thickness and cost, manufacturing tolerances and axial flexing of the walls has caused large variation in seal drag torque.